The Oath

I recently re-read Frank Peretti's thriller The Oath, and I couldn't help but notice parallels to Lovecraft's The Shadow Over Innsmouth. Both stories involve dark secrets kept by isolated communities who've made a pact with evil. The residents in both tales dislike nosy outsiders and refuse to speak to them. There have been many disappearances of locals and outsiders over the years in both novels. Old diaries and letters provide important sources of information in each. I don't think Peritti necessarily copied Lovecraft, but both authors used similar ingredients to create suspense. And I think The Oath would make an interesting CoC campaign.

I've read this book and although I can agree with your observations about the similarities between the towns I don't think The Oath has enough elements of cosmic horror in the same vein as Lovecraft to really make it lovecraftian. I'm sure someone could extrapolate something out of it to convert to a CoC campaign but I don't see the novel as is being a true CoC story. Don't get me wrong, loved the novel, but as for have enough cosmic elements of Lovecraft, I'd have to disagree.

The difference, I think, is the polar opposite worldviews of the authors. Lovecraft, the materialist, presents human existence as essentially hopeless -- a key part of his take on cosmic horror. For Peretti, the Christian, the horror is not that there isn't any hope but that people choose to destroy themselves with their bad decisions. They'd rather have their own way than survive.

You could potentially use the novel as a template to make a mythos adventure though. With Peretti's Christian motif, it ultimately is the triumph of good over evil, to make it more mythos you'd have to rewrite the overall plot of the inevitability of evil. Then instead of a dragon and the townsfolk, it could be the corruption of a local cult that runs the town has drawn some mythos (old or original) to the town. There's nothing in the mythos that I know of that would support people doing evil to create a creature like in The Oath, but you could have the corruption drawn some powerful mythos being to the area that the people will essential worship or think of some benefactor when instead it wants to destroy the town.

In a role-playing setting, of course, the triumph of good over evil would depend entirely on the PCs. In The Oath you essentially had a small cult consisting of immediate members of the Hyde family, which passed down beliefs and rites to the next male heir. The citizenry of Hyde Valley didn't need a Mythos critter to corrupt them; they were doing a pretty good job of it themselves.

Despite its different theological underpinnings, the Hyde River dragon shares several traits with the usual Lovecraftian entities: Most of the time it can't be seen. It has some sort of psychic/spiritual link with its victims and can draw them to it when the time is ripe. It is impervious to small arms fire, and even a massive quantity of dynamite can't kill it. It is intelligent and cunning and has an agenda undecipherable to those who revere it. It can be summoned but can't be controlled. Clues to its existence and habits are available in the form of old diaries, letters, court records, and newspaper clippings. And it can't be truly and permanently destroyed, at least not unless the townspeople have a change of heart.